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Battery Backup ICBattery Backup IC1. Background of Development3-1 Battery/Main Supply Changeover Circuit
Around 1985 when began the sales of IC for
1. 3.3V detect circuit ··· Main supply detection
microcomputer peripheral equipment and system
2. Transistor switch ····· Main supply ON/OFF switch
reset IC, a set manufacture inquired of us about IC for
3. Schottky diode ········ For supplying backup power
control of memory. It was a good change for us to
3-2 Configuration of CS (CE) Control circuit
decide a joint development of general purpose IC.
1. 4.2V detect circuit ········ Detection of main supply
This resulted in the development of PST531 and
2. Output reverse circuit ··· Reverses CS signal into
PST532. In 1989 they were improved into the backup
IC series dedicated to cards, games (popular-priced)
and to 1-chip microcomputers. Our IC series now
backup IC having the following additional functions.
includes low current consumption type products.3-3 Detection of Residual Voltage of Battery
1. 2.70V detect circuit ··· Equivalent to yellow alarm
2. What is Battery Backup IC?
Such IC memories as S-RAM and D-RAM all volatilize
2. 2.55V detect circuit ··· Equivalent to red alarm for
when power supply is cut off. Usually we use
batteries not to let these data disappear and to store
3-4 Battery Control Circuit
them for a longer time (of the order of several years).
1. This circuit controls so that battery power should
The battery backup IC can monitor main supply when
not be supplied even if the battery is replaced
storing data by means of battery and the like, control
when there remains no main power supply when
memory so that there should not arise any data
trouble in the memory, change the main supply over
to battery, and monitor the residual voltage of the
4. Principli of Operation 4-1 Battery/Main Supply Changeover Circuit
The 3.3V detect circuit turns off the transistor switch
3. Basic Configuration
when VCC is 3.3V or less and turns it on when the
Fig. 1 below illustrates the basic configuration of the
same exceeds 3.3V. The transistor ON will output the
voltage of main supply to VOUT and transistor OFF will
We have PST531, PST532 and MM1026 as IC of the
4-2 CS (CE) Control Circuit
The 4.2V voltage detect circuit makes CS signal to
low level and CS to high level when VCC is 4.2V or
lower, and CS to high level and CS to low level when
the same exceeds 4.2V. The CS signal level will
change over the S-RAM mode as indicated in Fig.1
When the CS signal is at low level (high level with CS
signal), that is when VCC is 4.2V or lower, then S-RAM
will enter into the backup mode and data read/write is
forbidden thus preventing data trouble.Fig.1 Basic ConfigurationBattery Backup IC5. Characteristics of Respective Models 5-1 MM10255-3 MM1027 and MM1081
Intended for D-RAM (no refresh control circuit) and
1. Two battery checkers built-in. Applicable to JEIDA
1. The power source used is Ni-Cd battery. Both main
Ver.4.0 specification for IC memory cards.
supply and backup supply incorporate stabilized
2. The main power supply provides power for IC,
while the backup power uses the lithium battery.
2. With main power supply detect circuit built-in, it
3. Built-in is the forced main power supply/backup
changes over the stabilized power supply and
power changeover circuit (low saturation realized
5-2 MM1026 and MM1080
4. Schottky barrier diode built-in on the backup side.
5. External transistor for output increase if connected,
1. Ideal for games and IC cards. Applicable to
6. Current consumption under normal conditions :
2. Then main power supplies the power for IC while
the lithium battery is used as backup power supply.
3. Incorporate forced main power supply/backup
supply changeover circuit by transistor switch (low
saturation realized by PNP transistor).
7. Current consumption under backup conditions :
4. Schottky barrier diode built-in on the backup side.
5. With the main power supply detect circuit built-in,
Fig. 3 shows a block diagram of MM1027.
6. Current consumption under normal conditions :
7. Current consumption under backup conditions :
Fig. 2 shows a block diagram of MM1026.Fig.3 Block Diagram of MM1027Fig.2 Block Diagram of MM1026Battery Backup IC5-4 MM1028 and MM1082 5-5 PST620 and PST621
For 1-chip microcomputers (backups as built-in
For 1-chip microcomputers (backups as built-in
1. The main power supply is 5V, while the backup
1. The main current uses 5V power supply, while the
backup power uses capacitor (super capacitor,
2. Built-in is the forced main power supply/backup
large capacitor, chemical capacitor). Under normal
power changeover circuit (low saturation realized
conditions the capacitor has been charged by 5V
power supply, which will back up the 1-chip
3. Back-up power control circuit by transistor switch
microcomputer as soon as the 5V power supply is
built-in on the backup side (low saturation realized
2. The voltage detector near 4.2V outputs CE signal
thereby switching the 1-chip microcomputer either
to standby or to normal mode. Further the low
MM1082 does not have this mode (because of low
voltage detector will detect 2 to 3V to reset the 1-
chip microcomputer. The inner 1-shot multivibrator
5. Current consumption under normal conditions :
will allow to reset for a given time when the voltage
6. Current consumption under backup conditions :
Fig. 4 shows a block diagram of MM1028.Fig.4 Block Diagram of MM1028Battery Backup ICMaintenance Tablecurrent OutputRepresentativeProduct Name OutputVoltage rank PackageConsumption currentInterchangeable Models
Note: This table does not guarantee complete interchangeability. Be sure to carry out sufficient technical
investigations, including for excess response, when changing.

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